The term attitude refers to the angular orientation of one coordinate system with respect to another. There are many practical reasons for determining attitude. For example, if one coordinate system is associated with an aerospace vehicle, and the other with some earth- or inertial-reference coordinates, this orientation can be used for the vehicle's guidance and flight control.
Attitude sensors are systems that are designed gather data used to determine the angular orientation of a vehicle such as a satellite. For spacecraft, attitude sensors detect (among other things) the limb of the earth, the location of the sun, the direction of Earth's magnetic field, and the direction of various stars. Global Positioning Systems have also been used for this purpose. Once the data is gathered, it is included with the known position and/or direction of these references in a relevant coordinate system and used in some sort of numerical algorithm that extracts a mathematical representation of the spacecraft's attitude. The most convenient form of attitude data is a collection of reference vectors—unit vectors in the direction of a known object or known objects. Since the 1960s, solutions to the problem of optimal attitude determination using many such measurements have been known, and data in this form is readily adapted to these classical algorithms.
One common type of attitude sensor used in satellite applications is the star tracker. A star tracker captures an image of stars that lie within its field of view. A computer resolves the location of each star as a unit vector from this focal-plane information, and these unit vectors are then combined to provide an attitude estimate. In general, the more data the better. Sun sensors measure only a single vector (the direction of the sun) but star trackers measure many more. Thus, star trackers can provide a more complete and accurate means of attitude determination.
One drawback to the use of star trackers is their complexity. Star trackers typically require significant computational power and an accurate, lengthy digital star catalog. The resulting system is expensive, complex, heavy and slow because a typical star tracker relies upon a large array of CCD elements from which digital images of the star clusters are captured. These digital images are then compared to star clusters in the digital star catalog. The processing power used to compare star clusters to the star catalog is significant, as it requires sophisticated pattern-recognition techniques.
All of these features make star trackers an expensive and sometimes problematic solution to attitude determination. Therefore, what is a needed is an attitude sensor system that avoids the complexity and costs of previous systems to provide a cost-effective means to determine vehicle orientation for systems such as ground-test facilitates, or satellite simulators, where incorporating a star tracker is too costly, or less practical than the alternatives.